The recordings can be produced with the aid of various recording methods:                Magnetic resonance tomography (MR angiography),        Computed tomography (CT angiography),        3D angiography with C-arm angiography devices and/or        X-ray projection (DSA, digital subtraction angiography).        
The first three methods are cross-sectional or volume imaging methods, and the fourth method is a 2D projection method.
In addition to the imaging of vessels during angiography, the visualization of contrast medium accumulation in tissue, for example in order to measure cerebral blood volume (CBV), has an increasing role to play.
Since the contrast medium spreads out dynamically within the tissue and the vascular system, the point in time at which the second recording, the contrast recording, is made is of central importance.
Owing to the relatively small amounts of contrast medium in the tissue, an observation by a human analyzer is more difficult and the operating experience on the basis of which the timing of the contrast recordings could be set is still limited.
The second recording of an angiography, the contrast image, is usually triggered manually by the operator of the device, for example a radiologist, as described for example in U.S. Pat. No. 4,483,342, or after a delay that can be varied according to the injection, as described in U.S. Pat. No. 4,581,635. The delay is in turn set by the operator on the basis of empirical values.
DE 32 15 552 C1 describes a method for 2D projection images in which the optimal mask image, which lies shortly before the rise of the contrast medium by one pixel along the time-contrast medium curve, and the optimal contrast image, the image with the maximum contrast medium density, are determined automatically. The points in time of the two sought images are found on the time-contrast medium density curve with threshold methods.
U.S. Pat. No. 4,581,635 A describes in general terms x-ray angiography systems and digital subtraction angiography. This does not include the triggering of recordings by automatic tracking of bolus arrival at a target location, which forms the main part of our invention.
U.S. Pat. No. 5,459,769A describes a method of determining the optimal start time for the recording, but this method differs from our invention as follows:    1. Bolus arrival is monitored manually. We have described a fully automatic method.    2. Bolus arrival is determined with a time-contrast curve by means of a user-defined region of interest (ROI), whereas our method takes the whole image into consideration, thus making automatic calculation possible.    3. As the recording is linked to (many) user interactions, the method in D2 cannot be considered to be a triggering, which is the main idea of our invention.